1. Field of the Invention
This invention relates to electronic calculators, particularly large calculators of the desk-top type for office use.
2. Description of the Prior Art
Conventional desk top calculators have their calculation and display components mounted in a single integral housing. The calculation components include a keyboard with data keys and function keys. Data (e.g. numerals 0 to 9) is entered into the calculator by operation of the data keys and the calculator processes the data in accordance with functions (e.g. +, -, .times., .div., etc.) selected by operation of the function keys. The tops of the keys usually have identification symbols corresponding to the data or functions controlled by the respective keys. The display components of the calculator include either a lighted display or a printer, or both, to display the entry of data, the calculation function carried out and the results of the calculation.
There are various types of desk top calculators; and they differ essentially according to the types of calculation functions which they perform. Basic calculators perform basic arithmetic functions such as addition, subtraction, multiplication and division. Other calculators, perform scientific functions including exponential, logarithmic and trigonometric functions. In addition, there are a number of specialized calculators available that perform specialized functions to fulfill the requirements of particular industries, such as bank teller and loan financing operations.
Although some calculation functions are common to different types of calculators, it is often necessary, to use different types of calculators in cases where substantially different types of calculations are to be carried out. This becomes quite costly because it requires the purchase of an entire new calculator with its own separate display components, for each general type of calculation function to be performed.
It is known to provide a programmable calculator whose calculation capabilities can be changed by substituting a new program. The calculator function program controls the operations carried out by the calculator in response to pressing the function keys; and, where a new program is substituted, the functions carried out when the function keys are pressed are different from what they were for the original program. Although the programmable calculator uses the same display components for different function programs, such programmable arrangements tend to be expensive and complex. Also, because the programs are changed without a corresponding key change, the key identification symbols do not correspond to the functions that the respective keys activate. This leads to operator confusion and error. Some use has been made of templates which are placed over the keys when the program is changed to indicate to the user what new functions are activated by the same keys. However, the use of templates is awkward; and, if the wrong template is installed, error or confusion will still occur.
Other attempts have been made to increase the versatility of desk type calculators without duplication of expensive printer components; and these other attempts have included the use of "stand-alone" printers which serve as output devices for interchangeable use with different calculators. Such printers serve as optional peripheral output devices and do not form an integral part of the separate calculators. Thus, each calculator still has its own output section with redundant fluorescent tube or other display device. Further, because the printer does not form an integral part of the output or display section of each calculator, the electrical interface between the calculator and the stand-alone printer is complex, requiring many connecting signal lines. A large number of interfacing signal lines is undesirable because it increases the chance of interface signal transmission error and breakdown. Examples of such shared printer systems are the Hewlett Packard Model HP-41C and Texas Instruments Model PC-100C devices. In the PC-100C system, different independently-operable hand-held calculators can be interchangeably connected to the optional printer unit. The calculator and printer housings do not, however, combine to form an integral calculator housing.
The Monroe Model 2850 desk-top calculator system represents another attempt to increase the versatility of a calculator without duplication of components. In the Monroe system, a snap-in key module (having from four to eight function keys) and corresponding electrical calculation circuits, is provided on the keyboard for optional functions. Interchanging one snap-in module for another snap-in module removes one set of optional functions and associated function keys (including corresponding key identification symbols) and substitutes another. Consequently, the function and keyboard modification available with this Monroe system is quite limited. Also, the calculator input section is not separable from the output section. The Monroe 2850 type calculator system thus does not permit the keyboard section to be separated from the display section for user positioning convenience during operation or for desk-drawer storage.